1. Technical Field
The present invention relates to an optical resonator suitably applied in the optical field, and to an optical sensor used in a fluid optical sensor employing an optical resonator.
2. Related Art
Ring shaped optical resonators are being actively researched into as means to realize sharp filters with extremely high wavelength selectivity (see; for example, Documents 1 and 6 listed below). For coupling light to the ring resonator a configuration is generally adopted where, for introducing light, an optical fiber or an optical waveguide path is disposed in the near vicinity of an optical waveguide path configuring the ring optical resonator. Research is also being actively pursued into optical resonators employing photonic crystals (see, for example, Document 2).
However, precise positioning is required in both ring shaped optical resonators and optical resonators employing photonic crystals, since light must be coupled to the optical waveguide path, leading to inferior ease-of-use.
In contrast, a Fabry-Perot resonator is superior from the standpoint of ease-of-use, since spatial light can be made directly incident to the optical resonator. Recently, as an application of a Fabry-Perot resonator, an optical resonator is disclosed having a structure in which fine pores are made in a wafer, and light is made to be perpendicularly incident on, and perpendicularly emitted from, the surface of the wafer (see, for example, Documents 3 to 5).
The technology disclosed in Documents 3 to 5 has a substrate made from Si or made from Al, with fine pores formed by employing an anode oxidation method. With this technology, application as sensor of an optical resonator and a variable wavelength filter is achieved by utilizing that facts that (1) a substance can be introduced into the optical resonator through the fine pores, and (2) the equivalent refractive index of the fine pores is capable of modulation by adjusting the volume ratio of the fine pores.
Furthermore, an optical resonator is being developed with a grating of concentric circle shape that makes light perpendicularly incident to, and perpendicularly emitted from, the face on which the optical resonator is formed (see, for example, Document 2).
However, in the technology disclosed in Documents 3 to 5, when the optical resonator is a micro-element, it is difficult to secure a long enough resonator length in order that diffraction occurs. As a result thereof, the wavelength peak of emitted light emitted due to resonance unfortunately becomes broad.
Furthermore, in the optical resonator disclosed in Document 7, the period of the grating needs to be half the wavelength of the emitted light, or less, leading to accompanying difficulties in production.
Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2007-183644
Document 2: U.S. Pat. No. 7,391,945
Document 3: U.S. Pat. No. 7,335,514
Document 4: U.S. Pat. No. 7,267,859
Document 5: U.S. Pat. No. 7,074,480
Document 6: Yasuo Kokubun, “High Index Contrast Optical Waveguides and Their Applications to Microring Filter Circuit and Wavelength Selective Switch”, IEICE transactions on Electronics, Vol. E90-C, No. 5 pp 1037-1045, 2007 May
Document 7: Xiankai Sun et. al., “Surface-emitting circular DFB, disk-, and ring-Bragg resonator lasers with chirped gratings: a unified theory and comparative study”, Optics Express Vol. 16, No. 12, pp 9155-9164, 2008, Jun. 9